Optimum laser surface treatment of fatigue damaged Ti–6Al–4V alloy

A laser surface treatment (LST), which enhances fatigue resistance of Titanium 6Al–4V alloy has been studied by 1.8 kW CW–CO2 Laser. The aim of the study is to find optimal conditions of treatment, which will extend the material resistance to fatigue failure, and explore the mechanisms involved. The LST was applied to ‘three point bend’ specimens loaded cyclically (R=0.1, 0.7σy near surface stresses), at different stages of fatigue life. Temperature surface fields were found by infrared camera. Two basic mechanisms were identified, one is related to healing mechanism (HM), which ‘erases’ prior fatigue damage up to a certain level, and the other is connected to microstructure mechanism (MM). Healing was found to be effective for surface temperatures above 400°C. In most cases, microstructure changes had a negative effect on fatigue resistance except for temperatures lower than 600°C and specific laser conditions. The combination of both mechanisms lead to optimal LST of 2 s and 0.85 kW cm−2, for which a 50% increase in fatigue life was found to be due to MM, in addition to a full healing. A positive correlation between hardness and fatigue life was also found.